1. Field of the Invention
The present invention relates to the preparation of anhydrides of carboxylic acids, and more particularly, to an improvement in the production of acetic anhydride by liquid phase carbonylation.
2. Background of the Invention
Acetic anhydride has been known as an industrial chemical for many years and large amounts are used in the manufacture of cellulose acetate. It has long been known that acetic acid and acetic anhydride can be prepared by the carbonylation of methanol and methyl acetate, respectively. Acetic anhydride has commonly been produced on an industrial scale by the reaction of ketene and acetic acid wherein the ketene was generated by high temperature dehydration of acetic acid or the thermal decomposition of acetone. Each of these "classic" processes has well-known drawbacks and disadvantages and the search for an improved process for the production of acetic anhydride has been a continuing one. Proposals for producing anhydrides by the carbonylation reaction of carbon monoxide upon various reactants using cobalt or nickel catalysts under very high pressure is well known. More recently, carbonylation at low pressures has been proposed in processes employing Group VIII noble metal catalysts with and without promoters. This process typically uses a rhodium compound and various promoters to convert a feed stream containing methanol, methyl acetate, dimethyl ether or mixtures thereof, and carbon monoxide to acetic anhydride. For example, U.S. Pat. No. 4,046,807 discloses a method for making acetic anhydride by contacting a noble metal containing catalyst, and preferably rhodium containing catalyst, with carbon monoxide and hydrogen. The '807 patent teaches that the presence of large quantities, that is from 5 to 50 volume %, hydrogen in the reaction gas is very favorable on the carbonylation reaction.
A problem with carrying out carbonylation using such noble metal catalysts and high concentrations of hydrogen is that the acetic anhydride formed is accompanied by ethylidene diacetate (EDA), acetaldehyde and acetic acid. Another problem with using high concentrations of hydrogen in the feed gas is that it increases the formation of methane. To reduce the EDA and methane production, U.S. Pat. No. 4,374,070 teaches liquid phase carbonylation of methyl acetate in the presence of rhodium an iodine compound and lithium wherein from about 2 to 7 volume % of the feed gas is hydrogen. The patent further teaches that the space-time yield is decreased considerably when the amount of hydrogen falls below 2 volume %.
Typically, carbonylation of methyl acetate or methyl ether carried out continuously in the presence of a Group VIII noble metal, with or without the presence of a promoter, further includes a halogen or halide such as methyl iodide to improve the carbonylation process. Volatile components of the reaction mixture are continuously separated from the relatively non-volatile Group VIII noble metal catalyst and the latter is continuously reused for further carbonylation. Typically, although not necessarily, the carbonylation takes place in a carbonylation zone and separation takes place by means of a flash distillation under a pressure lower than that prevailing in the carbonylation zone. Heat can be added or removed or the flash distillation can be carried out adiabatically, as is known to persons skilled in the art. In carrying out such processes where volatile components of the carbonylation mixture are continuously separated from the relatively non-volatile Group VIII noble metal catalyst, and the catalyst reused for further carbonylation, it has been discovered that the Group VIII noble metal catalyst gradually loses its activity and, after prolonged use, may become essentially inactive from a practical standpoint. It is, of course, possible to replace the catalyst at this point but this is an expensive operation even if the deactivated catalyst can be reclaimed.
To overcome this problem U.S. Pat. No. 4,252,741 issued to Porcelli et al. teaches that during the separation of volatile components from the Group VIII noble metal catalyst, a hydrogen partial pressure of at least 10 psi is maintained in a zone in which volatile components of the carbonylation mixture are continuously separated from the Group VIII noble metal. When the carbonylation is carried out in the presence of a metal promoter, such as chromium, it is also preferred to maintain a partial pressure of carbon monoxide of at least 15 psi in the separation zone.
U.S. Pat. No. 4,430,273 issued to Erpenbach et al. teach using a heterocyclic aromatic compound containing quaternary nitrogen and an aliphatic carboxylic acid having 1 to 8 carbon atoms in place of an organonitrogen compound or organophosphorus in a carbonylation process that uses Group VIII metal catalyst, an iodide compound and a promoter such as chromium, iron, cobalt, nickel and an organonitrogen compound or organophosphorus.
U.S. Pat. No. 4,661,631 issued to Becker et al. discloses a process for making acetic acid by the carbonylation of methanol in the presence of a catalyst comprising molybdenum-nickel or tungsten-nickel co-catalyst component in the presence of an iodide and in the presence of a promoter comprising an organo-phosphorus compound or an organo-nitrogen compound. Becker teaches that the rate of carbonylation wherein methanol is converted to acetic acid can be increased by carrying out the carbonylation with a mixture of hydrogen and carbon monoxide where a ratio of their respective partial pressures in the carbonylation zone is maintained at 0.05 to 0.4.
U.S. Pat. No. 4,735,749 issued to Fujiwa et al. disclose a process for maintaining the activity of the Group VIII metal catalyst used in carbonylation process for making an O-acetyl compound, such as acetic anhydride, where methyl acetate is reacted with carbon monoxide using a rhodium catalyst and an iodine compound. The process includes the step of regenerating the rhodium carbonylation catalyst by feeding the non-volatile, rhodium catalyst-containing solution, which is substantially free of any volatile components, into a treatment zone having a partial pressure of hydrogen of at least 0.2 kg/cm.sup.2 and heating the catalyst solution at a temperature from 100.degree. C. to 200.degree. C., for about 0.1 to about 2.0 hours. The regenerated catalyst is then re-circulated into the carbonylation reactor.
U.S. Pat. No. 4,994,608 issued to Torrence et al. disclose an improved process for carbonylation of methanol to acetic acid. Aside from the fact that the reaction is different from the one which the present invention is concerned, Torrence teaches converting an alcohol with carbon monoxide in the presence of a reaction medium comprising a rhodium catalyst, the ester of the alcohol being carbonylated with the acid product of the carbonylation reaction, a halide derivative of the hydrocarbon corresponding to the alcohol, especially the halide, and an iodide ion which is present in amount over and above the iodide which is present as the hydrocarbon halide, a water concentration of less than about 14 weight % and where the reaction medium further contains hydrogen in an amount sufficient to provide a hydrogen partial pressure of at least about 4.0 psi in the reactor.
U.S. Pat. No. 5,237,097 issued to Smith et al. discloses a method for solving the problem of rhodium-containing catalysts precipitating from the liquid phase recycle reactants present in a carbonylation process separation vessel. To prevent precipitation of the rhodium from the liquid catalyst, Smith disclose simultaneously adding with the liquid carbonylation product solution at the separation zone a carbon monoxide containing gas stream maintained at a partial pressure of up to about 30 psi.
Japanese patent publication 640998/1994 discloses a process for preparing acetic acid by reacting methanol with carbon monoxide in the presence of a rhodium containing catalyst and a metal iodide in a continuous process having no more than about 10 weight % water. The products are subjected to an evaporation step at a lower pressure than the carbonylation reaction and where the evaporation is carried out at a hydrogen partial pressure of at least 0.1 bar and/or the rhodium containing components are treated with hydrogen and carbon monoxide at a partial pressure of at least 0.1 bar.
It will be recognized that a common disadvantage of the aforementioned processes that treat the liquid phase rhodium-containing catalyst with hydrogen and/or carbon monoxide in a separate vessel where evaporation is simultaneously occurring is that much greater volumes of hydrogen and/or carbon monoxide are required to maintain the requisite partial pressures of the gases. Accordingly, there is still a need for a continuous carbonylation process that utilizes a Group VIII metal catalyst wherein the catalyst maintains its activity over a prolonged period of time without excessive losses of reactant materials.
It is therefore an object of the present invention to provide a further improved process utilizing carbonylation in the presence of a catalyst of the character just described.